Defective SCF-induced cell spreading of fyn^−/− BMMCs on fibronectin. (A) fyn^+/+, fyn^−/− BMMCs, and fyn^−/− BMMCs transduced with MSCV-Fyn were plated on fibronectin in the presence of SCF (20 ng/mL) for 15 or 45 minutes prior to fixation and F-actin staining as described in “Materials and methods.” Representative images obtained by fluorescence microscopy are shown. (B) Cell spreading and lamellipodia formation of SCF-treated BMMCs (20 ng/mL for 45 minutes) plated on fibronectin-coated wells were analyzed by light microscopy. Significant differences in cell spreading and lamellipodia formation in fyn^−/− BMMCs (indicated by an asterisk, P < .01, n > 100; error bars indicate standard deviations [SD]) with those of fyn^+/+ BMMCs. (C) fyn^−/− BMMCs showed significantly lower total cell spread area compared to wild-type (indicated by an asterisk; P < .01, n = 50/genotype; mean ± SD), as quantified using Image Pro Plus software (Media Cybernetics, Silver Spring, MD).